[PATCH v6 net-next 12/14] tcp: accecn: fallback outgoing half link to non-AccECN

chia-yu.chang
2025-11-14T07:14:21Z

According to Section 3.2.2.1 of AccECN spec (RFC9768), if the Server
is in AccECN mode and in SYN-RCVD state, and if it receives a value of
zero on a pure ACK with SYN=0 and no SACK blocks, for the rest of the
connection the Server MUST NOT set ECT on outgoing packets and MUST
NOT respond to AccECN feedback. Nonetheless, as a Data Receiver it
MUST NOT disable AccECN feedback.

Signed-off-by: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>

---
v3:
- Remove unnecessary brackets.
---
include/net/tcp_ecn.h | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/include/net/tcp_ecn.h b/include/net/tcp_ecn.h
index 57841dfa6705..42fd28d818aa 100644
--- a/include/net/tcp_ecn.h
+++ b/include/net/tcp_ecn.h
@@ -175,7 +175,9 @@ static inline void tcp_accecn_third_ack(struct sock *sk,
switch (ace) {
case 0x0:
/* Invalid value */
- tcp_accecn_fail_mode_set(tp, TCP_ACCECN_ACE_FAIL_RECV);
+ if (!TCP_SKB_CB(skb)->sacked)
+ tcp_accecn_fail_mode_set(tp, TCP_ACCECN_ACE_FAIL_RECV |
+ TCP_ACCECN_OPT_FAIL_RECV);
break;
case 0x7:
case 0x5:
--
2.34.1